6. CONCLUSION

We have come a long way in four lectures, from Einstein's misbegotten
introduction of the
cosmological constant at the beginning of the last century to its
triumphant return today.
Einstein's blunder is now seen as the key to understanding the very
beginning of the universe,
as represented by the theory of inflation, as well as the universe
today, dominated by
the mysterious dark energy that makes up more than two thirds of the
entire mass of the cosmos.
I have tried to convince you of two things: first, that the study of the
early universe
is particle physics in a very real sense, and second that
apparently
exotic theories of the early universe such as inflation (and perhaps
even elements of string
theory or some other variant of quantum gravity) are predictive and
testable. It is a difficult business, to be sure, compared to the clean
physics at, say, an e ± collider, but what we
learn about fundamental theory from cosmology is in many ways
complementary to the lessons learned from more traditional particle
physics.

Acknowledgments

I would like to give my warmest thanks to Harrison Prosper for
organizing a wonderful summer
school, and to the bright and motivated students who made lecturing
there a rare treat. I thank
Richard Easther and Brian Greene for helpful comments. ISCAP gratefully
acknowledges the generous support of the Ohrstrom foundation.